Wavelength conversion laser light source, laser light source device and two-dimensional image display device adopting the same, and method of setting temperature of wavelength conversion element

ABSTRACT

A wavelength conversion laser light source includes: an element temperature switching section that switches a temperature of the wavelength conversion element according to a harmonic wave output value as set in an output setting device, and the element temperature switching section for switching a temperature of a wavelength conversion element according to a harmonic wave output level as set in the output setting device, wherein the element temperature switch section includes an element temperature holding section that holds the wavelength conversion element at the temperature as switched by the element temperature switching section.

This application is a Divisional of application Ser. No. 12/328,131,filed Dec. 4, 2008 which is based on Japanese patent application serialNo. 2007-316895, filed in Japan Patent Office on Dec. 7, 2007, andJapanese patent application serial No. 2008-060653, filed in JapanPatent Office on Mar. 11, 2008, the contents of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The preset invention relates to a wavelength conversion laser lightsource for converting a laser beam emitted from a laser light sourceusing the non-linear optical effects, and to a laser light source deviceand a two-dimensional image display device adopting the same, and alsorelates to the method of setting temperature of a wavelength conversionelement provided in the wavelength conversion light source.

2. Description of the Background Art

Conventionally, a variety of wavelength conversion laser light sourceshave been developed and made into practical applications, wherein avisible laser beam is obtained, such as a green light or an ultravioletray which is obtained by further converting the green light, etc.,through the wavelength conversion using the non-linear optical effectsof a laser beam emitted from the Nd: YAG laser, or the Nd:YVO₄ laser,for example, as disclosed in Japanese unexamined Patent Publication No.2004/157217 and Japanese unexamined Patent Publication No. 2000/305120.These converted light beams are used for laser processing, or a laserdisplay, etc.

FIG. 1 shows a typical structure of a conventional wavelength conversionlaser light source using the non-linear optical effect. In order toobtain the non-linear optical effect, it is required to adopt thenon-linear optical crystals having the birefringence. Examples of suchnon-linear optical crystals having the birefringence include: LiB₃O₅(LBO: lithium triborate), KTiOPO₄ (KTP: Potassium Titanyl Phosphate),CsLiB₆O₁₀(CLBO: Cesium Lithium Borate); or LiNbO₃ (PPLN: LithiumTantalate), and LiTaO₃(PPLT: Lithium Tantalate) having a periodicalpolarization inversion structure, etc.

As shown in FIG. 1, a wavelength conversion laser light source 100includes a fundamental wave light source 101, a collective lens 108, anon-linear optical crystals (wavelength conversion element) 109, are-collimating lens 111, a wavelength-dividing mirror, a temperatureholder 116 such as a heater or the like for holding the temperature ofthe non-linear optical crystals constat, a control unit 115 forcontrolling a laser output, and a temperature controller 122 forcontrolling the temperature of the non-linear optical crystals providedin the control unit 115. For the fundamental wave light source 101, Nd:YAG laser, Nd:YVO₄ laser, fiber laser using Yb doped fiber having awavelength of 1.06 μm are generally used.

Here, the actual operations will be explained, which generate the secondharmonic wave having a wavelength of 0.532 μm which is around ½ ofwavelength (1.06 μm) of the fundamental wave.

The laser beam having a wavelength of 1.06 μm as emitted from thefundamental wave light source 101 is converged into the non-linearoptical crystals 109 by the collective lens 108. Here, the non-linearoptical crystals 109 needs to have the refractive index for the lighthaving the wavelength of 1.06 μm matched with the refractive index forthe light having the wavelength of 0.532 μm to be generated (phasematching condition). Generally, the refractive index for the crystalsvaries according to temperature conditions of the crystals. Therefore,the temperature of the crystals needs to be maintained constant. Forthis reasons, the non-linear optical crystals are placed in thetemperature holder 116, and are maintained at a predeterminedtemperature suited for the kind of the crystals. For example, whenadopting the LBO crystals, in order to obtain the type-1 non-criticalphase matching (the phase matching state), the LBO crystals need to bemaintained at a temperature in a range of from 148° C. to 150° C.

On the other hand, when adopting LiNbO₃ crystals having a periodicalpolarization inversion structure, it is possible to determine thetemperature and the wavelength for the phase matching condition byselecting the period for the periodical polarization inversionstructure. However, in order to maintain the phase matching condition,it is required to keep the element temperature of the wavelength of thefundamental wave constant.

However, it has been found that for some kinds of the non-linear opticalcrystals to be adopted as the wavelength conversion element, thetemperature of the element is raised by absorbing the fundamental waveand the harmonic wave as generated, which makes the phase matchingtemperature (wavelength) vary according to the output level of theharmonic wave, thereby presenting a problem in that a high conversionefficiency cannot be realized.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a wavelength conversionlaser light source, a laser light source device and a two-dimensionalimage display device adopting the same, and a method of setting thetemperature of a wavelength conversion element of the wavelengthconversion laser light source, which permit an efficient conversionirrespectively of changes in phase matching condition of the wavelengthconversion element according to an output level of the harmonic wave.

As described, a wavelength conversion laser light source, according toone aspect of the present invention includes: a fundamental wave laserlight source; a wavelength conversion element for converting afundamental wave emitted from the fundamental wave laser light sourceinto a harmonic wave, the wavelength conversion element being made of amaterial whose light absorption properties change according to an outputlevel of a harmonic wave; an output setting section for setting aharmonic wave output power level; and an element temperature switchingsection that switches a temperature of the wavelength conversion elementaccording to a harmonic wave output level as set in the output settingdevice, wherein the element temperature switch section includes anelement temperature holding section which holds the wavelengthconversion element at the temperature as switched by the elementtemperature switching section.

According to the foregoing structure wherein the wavelength conversionelement made of a material whose light absorption properties changeaccording to an output level of a harmonic wave, the phase matchingtemperature of the wavelength conversion element changes according to anoutput level of the harmonic wave. In response to changes in phasematching temperature of the wavelength conversion element according tothe output level of the harmonic wave, the element temperature switchingsection switches the element temperature of the wavelength conversionelement according to the output level of the harmonic wave, and theelement temperature holding section holds the wavelength conversionelement at the element temperature as switched. As a result, it ispossible to realize the wavelength conversion laser light source, whichpermits an efficient conversion without being adversely affected bychanges in phase matching condition of the wavelength conversion elementaccording to the output level of the harmonic wave.

Furthermore, the wavelength conversion element can be maintained at adesirable temperature corresponding to the output level of the harmonicwave, thereby realizing a high wavelength conversion efficiently.

These and other objects, features and advantages of the presentinvention will become more apparent upon reading the following detaileddescription along with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an explanatory view schematically showing the structure of aconventional wavelength conversion laser light source;

FIG. 2 is an explanatory view schematically showing the structure of awavelength conversion laser light source in accordance with oneembodiment of the present invention;

FIG. 3 is a graph showing the relationship between the fundamental waveinput and the second harmonic wave output, and the relationship betweenthe fundamental wave input and a shift in temperature from a phasematching temperature of a wavelength conversion element when adoptingthe wavelength conversion laser light source in accordance with oneembodiment of the present invention;

FIG. 4 is a graph showing the relationship between the second harmonicwave output and a shift in temperature from a phase matching temperatureof a wavelength conversion element with a parameter of fundamental waveinput when adopting the wavelength conversion laser light source inaccordance with one embodiment of the present invention;

FIG. 5 is a graph showing the relationship between the fundamental waveinput and the second harmonic wave output with a parameter of a shift intemperature from a phase matching temperature of a wavelength conversionelement when adopting the wavelength conversion laser light source inaccordance with one embodiment of the present invention;

FIG. 6 is a graph showing the relationship between a shift intemperature from a phase matching temperature of a wavelength conversionelement and a second harmonic wave output with a parameter offundamental wave input when adopting the wavelength conversion laserlight source in accordance with one embodiment of the present invention;

FIG. 7 is a block diagram showing schematic structures of a control unitand a temperature controller of a wavelength conversion element providedwith a wavelength conversion laser light source in accordance with oneembodiment of the present invention;

FIG. 8 is a flowchart showing processes of controlling a wavelengthconversion element by a temperature controller in accordance with oneembodiment of the present invention in normal state;

FIG. 9 is a flowchart showing processes of controlling a wavelengthconversion element by a temperature controller when executing theelement temperature learning processes in accordance with still anotherembodiment of the present invention;

FIG. 10 is a graph showing the relationship between the fundamental waveinput and the second harmonic wave output with a parameter of a shift intemperature from a phase matching temperature of a wavelength conversionelement in accordance with one embodiment of the present invention;

FIG. 11 is a flowchart showing processes of controlling a wavelengthconversion element by a temperature controller in accordance with oneembodiment of the present invention when executing the elementtemperature learning processes;

FIG. 12 is a graph showing differences in conversion efficiency from afundamental wave to a harmonic wave between when adopting a temperatureleaning method in accordance with one embodiment of the presentinvention and when adopting a temperature leaning method in accordancewith another embodiment of the present invention;

FIG. 13 is a graph showing the relationship between a shift intemperature from a phase matching temperature of a wavelength conversionelement and a second harmonic wave output generated at optimal elementtemperature with respect to the fundamental wave input in accordancewith still another embodiment of the present invention;

FIG. 14 is a flowchart showing processes of controlling a wavelengthconversion element by a temperature controller in accordance with stillanother embodiment of the present invention in normal state;

FIG. 15 is a flowchart showing processes of controlling a wavelengthconversion element by a temperature controller when executing theelement temperature learning processes in accordance with still anotherembodiment of the present invention;

FIGS. 16A to 16C are explanatory views which depict harmonic wave outputpulse waveform based on which fine adjustments in temperatures of awavelength conversion element are to be made in accordance with stillanother embodiment of the present invention;

FIG. 17 is a flowchart showing processes of making fine adjustment intemperatures of a wavelength conversion element based on pulse waveformin accordance with still another embodiment of the present invention;

FIG. 18A is a graph showing the relationship between the second harmonicwave output level (wavelength of 532 nm) with respect to the fundamentalwave (wavelength of 1064 nm) in accordance with still another embodimentof the present invention;

FIGS. 18B and 18C are explanatory views showing the beam shape inaccordance with still another embodiment of the present invention;

FIG. 19A is an enlarged view schematically showing an abnormality inbeam diameter determining mechanism in accordance with still anotherembodiment of the present invention;

FIG. 19B is an explanatory view, which depicts a schematic structure ofa photoreceptor in accordance with a still another embodiment of thepresent invention;

FIG. 19C is an explanatory view, which explains a mechanism of detectingan abnormality in beam diameter when adopting a photoreceptor inaccordance with still another embodiment of the present invention;

FIG. 20A is a block diagram schematically showing a structure of acontrol unit of a mechanism of detecting an abnormality in beam diameterin accordance with still another embodiment of the present invention;

FIG. 20B is a flowchart showing control processes by a control unit of amechanism of detecting an abnormality in beam diameter in accordancewith still another embodiment of the present invention;

FIG. 21A is an explanatory view, which depicts one example of aphotoreceptor of a mechanism of detecting an abnormality in beamdiameter in accordance with still another embodiment of the presentinvention;

FIG. 21B is an explanatory view, which depicts another example of aphotoreceptor of a mechanism of detecting an abnormality in beamdiameter in accordance with still another embodiment of the presentinvention;

FIG. 21C is an explanatory view, which depicts still another example ofa photoreceptor of a mechanism of detecting an abnormality in beamdiameter in accordance with still another embodiment of the presentinvention;

FIG. 22A is an explanatory view schematically showing a structure of acontrol unit of a mechanism of detecting an abnormality in beam diameterin accordance with still another embodiment of the present invention;

FIG. 22B is a flowchart showing control processes by a control unit of amechanism of detecting an abnormality in beam diameter in accordancewith still another embodiment of the present invention;

FIG. 23 is an explanatory view schematically showing the structure of aprojector (projection display) adopting a laser light source inaccordance with still another embodiment of the present invention;

FIG. 24A is an explanatory view schematically showing an examplestructure of a liquid crystal display adopting a laser light source ofthe present invention;

FIG. 24B is a cross-sectional view of the liquid crystal display of FIG.24A;

FIG. 25 is an explanatory view schematically showing an examplestructure of a laser light source provided with fiber adopting the laserlight source of the present invention;

FIG. 26A is a graph showing the relationship between a light intensityand a beam diameter directly before being connected to atransmission-use fiber when adopting a beam diameter change mechanism ofFIG. 25; and

FIG. 26B is a graph showing the relationship between a light intensityand a transmission-use fiber NA when adopting a beam diameter changemechanism of FIG. 25.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings.

In the first through third embodiments, a method of obtaining thetemperature of the wavelength conversion element and setting the elementtemperature at a predetermined temperature value will be explained. Inthe fourth embodiment, a method of determining a timing of re-settingthe temperature of the wavelength conversion element at an optimaltemperature in consideration of changes in temperature of the wavelengthconversion element as time passes, and a method of making fineadjustments on the temperature of the wavelength conversion element.

First Embodiment

The following descriptions will explain a wavelength conversion laserlight source in accordance with one embodiment of the present inventionwith reference to figures.

In the present embodiment, the method of switching a temperature atwhich a wavelength conversion element 209 is held according to afundamental wave power level with which a harmonic wave of the outputpower level as set in an output setting device 201 can be obtained.

As shown in FIG. 2, a wavelength conversion laser light source 200 inaccordance with the present embodiment includes a fundamental wave lightsource 231, a first dichroic mirror 236, a second dichroic mirror 237, acollective lens 208, a non-linear optical crystals (wavelengthconversion element) 209, a re-collimating lens 211, a photoreceptor(photodiode) 212, a beamsplitter 213, etc.

For the fundamental wave light source 231, adopted is a fiber laserlight source adopting a Yb doped fiber 233. It is advantageous to adopta fiber laser light source in that the oscillation wavelength and thespectrum width can be determined as desired. Therefore, when adoptingsuch fiber laser light source, it is possible to significantly improvethe conversion efficiency from a fundamental wave into a harmonic waveby reducing the spectrum width.

The fundamental wave 235 generated by the fundamental wave light source(fiber laser light source) 231 is collected into the non-linear opticalcrystals (wavelength conversion element) 209 by the collective lens 208.In the present embodiment, MgO: LiNbO₃ crystal element (MgLN element) isadopted as the non-linear optical crystals, wherein the periodicalpolarization inversion structure is formed.

The wavelength conversion laser light source 200 in accordance with thepresent embodiment is provided with the temperature holder 216 under thelower surface of the wavelength conversion element (non-linear opticalcrystals) 209. This temperature holder 216 serves to maintain thewavelength conversion element 209 at a predetermined holdingtemperature. For the temperature holder 216, adopted is a Peltierdevice.

The second harmonic wave having wavelength converted by the wavelengthconversion element 209 is formed into a parallel beam by there-collimating lens 211. After having formed into the parallel beam, thebeam is separated by the beamsplitter 213 into a fundamental wave whichhas not been converted to the harmonic wave, and the harmonic wave asbeing converted.

The laser output is controlled with current supplied to the pump lightsource of the fundamental wave light source 231. Incidentally, suchmethod of controlling the fundamental wave input may be adopted, whereina part of the fundamental wave may be taken out directly before thefundamental wave is incident into the wavelength conversion element 209,to monitor the incident light into the wavelength conversion element209.

In the structure of FIG. 2, a beam splitter 213 and a photoreceptor(photodiode) 212 are provided before and after the wavelength conversionelement 209. However, it may be arranged so as to provide the beamsplitter 213 and the photoreceptor 212 either before or after thewavelength conversion element 209.

FIG. 3 is a graph showing the relationship between the fundamental waveinput and the second harmonic wave output, and the relationship betweenthe fundamental wave input and a shift in temperature Δt (° C.) from aphase matching temperature T (° C.) of the wavelength conversion element209 with a power of the input fundamental wave of 500 mW when adoptingthe wavelength conversion laser light source 200 in accordance with thepresent embodiment.

As is clear from the graph of FIG. 3, as the second harmonic wave outputincreases, the phase matching temperature becomes lower. When the outputpower for the second harmonic wave is increased to 2.5 W, the phasematching temperature is reduced from the temperature initial value by1.3 (° C.).

FIG. 4 is a graph showing the relationship between the second harmonicwave output (W) and a shift in temperature Δt (° C.) from a phasematching temperature T (° C.) of the wavelength conversion element 209with a parameter of fundamental wave input of 500 mW.

As is clear from the graph of FIG. 4, the peak value of the curve, i.e.,a shift in temperature Δt from the phase matching temperature T (° C.)of the wavelength conversion element 209 is shifted to the lowertemperature side as the output power level for the second harmonic wave(W) is increased as in case of the graph of FIG. 3.

FIG. 5 is a graph showing the relationship between the fundamental waveinput and the second harmonic wave output with a parameter of a shift intemperature Δt from a phase matching temperature T ('C) of thewavelength conversion element 209 with a power of the input fundamentalwave of 500 mW. As can be seen from the graph of FIG. 5, in some holdingtemperature range for the wavelength conversion element 209, the outputpower of the harmonic wave is increased as the power of the fundamentalwave increases. However, in other holding temperature ranges, the outputpower of the harmonic wave is decreased as the power of the fundamentalwave increased from a certain point for the input power of thefundamental wave. Specifically, when a shift in temperature Δt (° C.)from the phase matching temperature T(t) of the wavelength conversionelement 209 with a power of the input fundamental wave of 500 mW is Δt(t)=−1.0(t), the output power for the harmonic wave is exponentiallyincreases with an increase in the input power for the fundamental wave.On the other hand, when a shift in temperature it (° C.) from the phasematching temperature T(° C.) of the wavelength conversion element 209with a power of the input fundamental wave of 500 mW is Δt (° C.)=0 (t),the output power for the harmonic does not increase with an increase inthe input power for the fundamental wave with a boundary of the outputpower of the second harmonic wave of 5.5 W. When adopting the laserlight source, it is general to control the output power from the lightsource to be constant (Auto Power Control: APC).

When carrying out the auto power control, an increase or a decrease inoutput power of the light source should correspond to increase ordecrease in current applied. However, it has being found by theinventors of the present application that a decrease in output power ofthe second harmonic wave raises the problem not only in that anefficient conversion cannot be performed but also in that the outputpower for the second harmonic wave cannot be increased with an increasein input power of the fundamental wave under the auto power control,namely, the output power cannot be controlled.

It was found by the inventors of the present application that suchproblem of a decrease in output power is outstanding when adoptingnon-linear optical crystals having light absorbing properties such asMg:LiNbO₃, Mg:LiTaO₃, KTiOPO₄, two-photon absorption in particular whichis excited by a laser beam.

In response, the output power for the harmonic wave may be adjusted asdesired by adjusting temperature the wavelength conversion element 209.However, when adopting the non-linear optical crystals as the wavelengthconversion element 209, under some conditions, the wavelength conversionelement 209 absorbs in an excess amount of harmonic wave output at themoment the phase matching state is realized. Further, heat generatedfrom the wavelength conversion element by absorbing the harmonic waveoutput, may result in the problem of damage in wavelength conversioncrystals.

In view of the foregoing problems, the present and subsequentembodiments will discuss the method of preventing an occurrence of suchevent that the output power of the harmonic wave becomes out of controldue to the properties of absorbing light of the wavelength conversionelement 209.

The graph of FIG. 6 shows the relationship between a shift intemperature Δt (° C.) from the phase matching temperature T (° C.) ofthe wavelength conversion element 209 with a power of the inputfundamental wave of 500 mW and an output power of the second harmonicwave with a parameter of fundamental wave input.

According to the method of the present embodiment, a peak search iscarried out to obtain a temperature of the wavelength conversion element209 at which the output power for the harmonic wave is maximizedaccording to a difference in input power of the fundamental wave. Then,the resulting optimal temperature of the wavelength conversion element209 is stored in the EEPROM (ELECTRICALLY ERASABLE PROGRAMMABLEREAD-ONLY MEMORY) 706. The optimal temperature as stored in the EEPROM706 is read to be switched from the currently set temperature of thewavelength conversion element 209 when next setting the output power forthe output setting device 201.

The respective cases with different change points with output powers of1 W, 1.6 W, and 2.2 W (corresponding to input power of the fundamentalwave of 4 W, 5.5 W and 7.0 W) will be explained with reference to FIG.6.

Firstly, the information indicative of that the output powers 1 W, 1.6 Wand 2.2 W of the second harmonic wave correspond to input powers 4 W,5.5 W and 7.0 W of the fundamental wave respectively, and theinformation indicative of that respective temperatures of the wavelengthconversion element 209 corresponding to the output level (1 W, 1.6 W and2.2 W) of the second harmonic wave are A° C., B° C. and C° C. arerecorded in the EEPROM 706 when shipping from the factories, in the formof input current to the fundamental wave light source 231.

When inputting X(W) in the output setting device 201 as a desired outputpower for the second harmonic wave, the temperature of the wavelengthconversion element 209 is set under the following conditions.

-   -   X<1:A° C.    -   1≦X<1.6:B° C.    -   1.6≦X<2.2:C° C.    -   2.2≦X: temperature is not set and re-input is required.

When the holding temperature of the wavelength conversion element 209 isbeing changed to a desired output power as inputted, a caution signal isoutputted to inform the user that the temperature is being adjusted.When the wavelength conversion element 209 is reached to the holdingtemperature as set, it is set in a stand-by state to be ready for theoutput of a harmonic wave laser.

In response to an instruction from the output setting device 201, aharmonic wave is outputted from the wavelength conversion laser lightsource 202.

Here, while the harmonic wave is being emitted, a change in targettemperature of the wavelength conversion element 209 may be required inresponse to a change in set output power of the harmonic wave by theoutput setting device 201. In this case, based on the output power valueof the harmonic wave as re-set, the target temperature of the wavelengthconversion element 209 is changed under the above conditions. In thiscase, to avoid fluctuations in harmonic wave output while thetemperature is being adjusted, the auto power control is performed byadjusting a current applied to the pump LD, to reduce functions inoutput power.

According to the present embodiment, when the target output power cannotbe changed by controlling the current applied to the pump LD, a cautionsignal is outputted from the control unit 225 to inform the user thatthe target output power is being changed.

FIG. 7 is a block diagram showing schematic structures of the controlunit 255 and the temperature controller 711 of the wavelength conversionelement 209 provided with the wavelength conversion laser light sourcein accordance with the present embodiment.

As shown in FIG. 7, the temperature controller 711 includes a powersupply 708, a thermistor 703, an A/D converter 704 for converting atemperature signal from the thermistor 703 into a digital value, aregister 705 storing a temp. signal as converted into a digital value bythe A/D converter 704, EEPROM 706 for storing a table of temperatures ofthe wavelength conversion element 209 corresponding to respective outputpowers of the harmonic wave and current required, an MPU 707 to whichthe data indicative of a set value for the output power of the harmonicwave from the control unit 225 is transferred from the control unit 225,and a switch 709 which controls the PWM (Pulse Width Modulation) withrespect to the current wavefrom to be supplied to the temperature holder216 from the power supply 708.

In the present embodiment, the temperature holder 216 is controlled inthe following manner. That is, the information indicative of thetemperature of the wavelength conversion element 209 corresponding tothe output level (output power level) of the harmonic wave as stored inthe EEPROM 706 is obtained. Then, the MPU 707 compares and computes thetemperature of the wavelength conversion element 209 as obtained withthe temperature present value stored in the register 705. As a result,the temperature holder 216 is controlled based on the current to beapplied to the temperature holder 216 from the power supply 708 in viewof the polarity and the waveform of the current under the PWM (PulseWidth Modulation) control by giving instructions to the switch 709.

The wavelength conversion element 209 is placed on the temperatureholder 216, and the temperature of the wavelength conversion element 209is monitored indirectly by monitoring the temperature of the temperatureholder 216. The temperature signal from the thermistor 703 is convertedinto a digital value by the A/D converter 704 to be stored in theregister 705. The temperature signal from the thermistor 703 isconverted into a digital value by the A/D converter 704 to be stored inthe register 705. In the EEPROM 706, stored beforehand together withinput currents is a table for the temperatures of the wavelengthconversion element, which respectively correspond to the output powersof the harmonic wave. The set value for the output power for theharmonic wave is transferred from the control unit 225 to the MPU 707.Then, the element temperature corresponding to the set value for theoutput power as transferred is obtained from the EEPROM 706. The MPU 707compares and computes the element temperature thus obtained with respectto the element temperature present value stored in the register 705. Asa result, the temperature holder 216 is controlled based on the currentto be applied to the temperature holder 216 from the power supply 708 inview of the polarity and the waveform of the current under the PWM(Pulse Width Modulation) control by giving instructions to the switch709.

FIG. 8 is a flowchart showing processes of controlling a wavelengthconversion element by a temperature controller in normal state inaccordance with the present embodiment. FIG. 9 is a flowchart showingprocesses of controlling a wavelength conversion element by atemperature controller when executing the element temperature learningprocesses in accordance with the present embodiment.

When starting the normal driving, the temperature initial value T of thetemperature holder 216 for the wavelength conversion element 209 isobtained (S1). Then, the output power set value Psv as set in the outputsetting device 201 is obtained from the control unit 225 (S2). Based onthe set Psv, the Tsv is obtained from the EEPROM 706 (S3), to be set asthe target element temperature (S4). Before starting the temperaturecontrol, the temperature present value Tpv of the wavelength conversionelement 209 is obtained from the thermistor 703 (S5). Then, the polarityof current to be applied to the temperature holder 216 (in the case ofadopting the Peltier device as the temperature holder 216) and thewaveform are subjected to control.

Then, a coefficient G is computed for use in controlling the currentwaveform based on the temperature initial value T, the temperature setvalue Tsv and the temperature present value Tpv (S6).

G=(Tpv−T)/(Tsv−T)  (1)

In the present embodiment, the duty ratio is switched based on thecoefficient G thus computed in S6 to control the current waveform.

When the condition of 0.9≦G is satisfied (YES in S7), the PWM control isperformed (S8). On the other hand, when the condition of 0.9−G is notsatisfied (NO in S7), the duty ratio is set to 100%, and the PWM controlis not performed (S9).

In replace of the foregoing structure of switching the duty ratio basedon the coefficient G, it may be arranged so as to directly use thecomputed value from the equation of the current duty ratio forcontrolling temperature=b*(1−G)/(b+G) using the coefficient G and thecoefficient b (0<b≦1).

Next, it is then determined if a change in current polarity is requiredby the following inequality (2) using Tpv, Tsv and a coefficient a(S10).

Tpv>a×Tsv  (2)

When the above inequality (2) holds (YES in S10), the polarity ofcurrent is changed (S11). On the other hand, when the condition of 0.9≦Gis not satisfied is not satisfied (NO in S7), it is determined if theabove inequality (2) holds without performing the PWM control in S8(S10).

The coefficient a in the inequality (2) is adopted for preventing thepolarity of the current to be switched to the normal state frequently.It is preferable that the coefficient a falls in a range of from 1.1 to1.2.

Incidentally, when Tpv is in a range of 1.1 Tsv to 1.2 Tsv, the currentmay be cut to reduce the temperature of the wavelength conversionelement 209 by natural cooling.

After the polarity of current changes in S11, the sequence goes back toS5 where the current temperature value of the wavelength conversionelement 209 is obtained (S5). By repeating the foregoing processes, agreen light can be outputted under stable condition.

Next, the processes for controlling the wavelength conversion element bya temperature controller will be explained in reference to the flowchartof FIG. 9. In the present embodiment, the temperature learning processesrefer to the processes of re-setting the target holding temperature ofthe wavelength conversion element to an optimal temperature for thedesired output power of the harmonic wave, as such target holdingtemperature is subjected to change as time passes.

Upon starting the element temperature learning processes, first theoutput power set value Psv for leaning is obtained from the EEPROM 706(S21). The temperature set value Tsv is determined based on the outputpower set value Psv for learning (S22). For the element temperaturelearning processes of the wavelength conversion element 209, a phasematching temperature of the wavelength conversion element 209 issearched based on peak hold by scanning temperatures. Therefore, theinitial temperature set value is set to 0.5 (° C.) below Tsv (S23).

Subsequently, the harmonic wave is actually generated to obtain theharmonic wave output Ppv₁, to be stored in the register 705 (S24). Thetemperature set value Tsv is then increased by 0.05 (° C.) to bereplaced with the currently set temperature (S25).

Next, the harmonic wave output Ppv_((n+1)) of the nth loop is obtainedto be temporarily stored in the register 705 (S26). The harmonic waveoutput Ppv_((n+1)) is compared with the harmonic wave output Ppv_(n)(S27).

Ppv _(n) >Ppv _((n+1))  (3)

When the above inequality (3) does not hold (NO in S27), the temperatureset value Tsv is increased again by 0.05 (° C.) to be replaced with thecurrently set temperature repetitively. On the other hand, if the aboveinequality (3) holds (YES in S27), the sequence goes out of the loopand, the temperature set value Tsv is then reduced by 0.05 (° C.) to bestored in the EEPROM 706 as an optimal temperature value (S28), therebyterminating the foregoing element temperature learning processes.

In the foregoing element temperature leaning processes, the initialtemperature set value is set to 0.5 (° C.) below Tsv. However, how manydegrees to be decreased from the Tsv is to be determined based on thetolerance range for temperatures (temperature range at which the outputlevel is ½) of the wavelength conversion element 209, i.e., half widthat half maximum, HWHM from the peak to the ½ of the output level.

Here, in order to prevent the misreading of the sub-peak in thetemperature tuning curve of the harmonic wave output as the peak, it isdesirable that a drop in temperature be set to 0.5 to 1 times of thetolerance range for the temperature Δt of the wavelength conversionelement 209.

As shown in the graph of FIG. 5, up to the range for the harmonic waveof 1.0 W, the harmonic wave output increases monotonically with respectto the fundamental wave input. On the other hand, in the range for theharmonic wave above 1.0 W, a problem arises in that the auto powercontrol cannot be performed at the temperature of the wavelengthconversion element set to the phase matching temperature if thetemperature of the wavelength conversion element is subjected to changedue to some disturbances as shown in the graph of FIG. 5. Whengenerating the harmonic wave with output power above 1.0 W, it istherefore desirable to set the target temperature to 0.2° C. below thephase matching temperature as obtained from the peak search.

In the case of adopting the periodically poled MgLN element of 25 mm inlength as a wavelength conversion element 209, and converting collectingthe light at the center of the element in the confocal condition, it isdesirable to set the target temperature below the phase matchingtemperature by 0.3 (° C.) to 0.05 (° C.) for the following reason: Thatis, the temperature tuning curve is shifted to the lower temperatureside with respect to the fundamental wave input (FIG. 4). Namely, if thetarget temperature is set to the phase matching temperature, the outputpower for the harmonic wave cannot be increased with an increase ininput power of the fundamental wave. It is therefore effective to shiftbeforehand the target temperature to the low temperature side.

Here, how many degrees to be decreased from the phase matchingtemperature should vary according to various factors such as the lengthof the wavelength conversion element 209, the tolerance range fortemperatures, focal conditions of the fundamental wave (beam waistdiameter, focal position). Specifically, the temperature set value canbe shifted to a lower temperature side from the phase matchingtemperature, for example, by increasing the length of the wavelengthconversion element 209, increasing the beam diameter, or moving thefocal position of the fundamental wave closer to the light incidentsurface of the element.

Incidentally, a timing at which the element temperature leaningprocesses are to be executed may be set automatically, for example, bythe following method. That is, directly before and after driving thedevice, the capacity left to the upper limit current to be applied tothe pump LD is detected and recorded in the EEPROM 706.

For example, the element temperature learning processes may be carriedout when the following equation (4) is satisfied.

I ₁×(1+d/100)=I _(lim)  (4).

In the equation (4), I₁ indicates the required LD current for obtainingthe output P₁, d (%) is a margin left to the limit value I_(lim) to beapplied to the pump LD and the auto power control.

Here, a program may be set to instruct a computer to automatically carryout the temperature learning processes when it is determined that theelement temperature learning processes are to be carried out whileinforming the user of that the device is being in the elementtemperature learning processes. Alternatively, it may be arranged suchthat the user operates the device to execute the temperature learningprocesses in response to a caution to do so.

In any case, the device cannot be used while the temperature learningprocesses are being executed. It is therefore desirable that theinformation indicative of being in the processes of learning temperatureis displayed using the operation panel, and that the device is set in alocked state except for the case where the user needs to operate to shutdown the device to execute an automatic emergency shutdown system.

Second Embodiment

The following descriptions will explain another embodiment of thepresent invention with reference to figures.

In the present embodiment, a peak search is to be executed for anoptimal temperature of a wavelength conversion element 209 for outputpowers set to 10% to 20% above the output power that can be set in theoutput setting device 201. Namely, in the present embodiment,explanations will be given through the case of using the results ofsearch performed in the entire range for output powers.

The basic structures for the laser light sources with fiber inaccordance with the present embodiment and the subsequent embodiments tobe explained later are the same as that of the first embodiment. In thepresent and subsequent embodiments, explanations will be given mainly onthe characteristic structures for each embodiment, and explanations onthe members having the same structures and the functions shall beomitted by designating the same reference numerals for convenience forexplanations.

In the present embodiment, the required harmonic wave output value isset to 2.2 W, and the peak search is to be carried out for the outputpower of the harmonic wave of 2.6 W, which is approximately 1.2 times ofthe required output power (2.2 W).

FIG. 10 is a graph showing the relationship between the fundamental waveinput and the second harmonic wave output (w) with a parameter of ashift in temperature Δt (° C.) from a phase matching temperature T(° C.)of a wavelength conversion element 209 with a power of the inputfundamental wave of 500 mW.

In the present embodiment, the harmonic wave with the output power of2.6 W could be obtained with Δt=−1.4 (° C.) by carrying out a peaksearch with the input fundamental wave power of 8.5 W. Then, thetemperature of the wavelength conversion element 209, i.e., T-1.4° C. isrecorded in the EEPROM 706, and it is controlled to adjust thetemperature of the wavelength conversion element 209 at T-1.4° C. whendriving the device.

With an increase in input power of the fundamental wave, the outputpower of the harmonic wave can be increased monotonically with respectto the fundamental wave input in the output power of the harmonic waverange of 0 W to 2.6 W. Then, the harmonic wave output can be controlledwith the auto power control based on the current to be applied to thepump LD. Here, by setting the output power of the harmonic wave, forexample, to 2.2 W, when actually driving the device, it is possible tomaintain the output power to the desired level with the auto powercontrol only by increasing the input power of the fundamental wave inview of changes in environment. In this case, although it is not shownin the graph of FIG. 10, it is appropriate to set the output power ofthe harmonic wave to 2.2 W when actually driving the device includingthe control margin, i.e., the output power that can be set in the outputsetting device 201 in view of the fact that the output power of theharmonic wave will be saturated at above the output power of theharmonic wave of 2.6 W.

Next, the processes for learning the temperature of the wavelengthconversion element 209 will be explained to FIG. 11. FIG. 11 is aflowchart showing processes of controlling a wavelength conversionelement when executing the element temperature learning processes.

In the present embodiment, the current I_(p=2.6W) required for obtainingthe harmonic wave with the output power of 2.6 W is recorded in theEEPROM 706 (FIG. 7) together with the initial temperature for thewavelength conversion element 209. Upon starting the element temperaturelearning processes, first, the current I_(p=2.6W) for learning requiredfor obtaining the harmonic wave with the output power of 2.6 W isobtained from the EEPROM 706 (S31). In the meantime, the initialtemperature (Tsv₀) for learning is obtained from the EEPROM 706 (S32).Then, the scanning of temperatures is carried out with the temperatureset value of the wavelength conversion element 209, which is set to 0.5(° C.) below Tsv₀ (S33). Thereafter, the harmonic wave output Tsv₁ isactually generated, to be stored in the register 705 (S34).

Then, the temperature set value Tsv is then increased by 0.05 (° C.) tobe replaced with the currently set temperature (S35).

Next, the harmonic wave output Ppv_((n+1)) of the nth loop is obtainedto be temporarily registered in the register 705 (S36). The harmonicwave output Ppv_((n+1)) is compared with the harmonic wave output Ppvn(S37). If the following inequality (5) is satisfied (YES in S37), thetemperature set value Tsv is increased again by 0.05 (° C.) to bereplaced with the currently set temperature repetitively.

Ppv _(n) <Ppv _((n+1))  (5).

On the other hand, when the above inequality does not hold (NO in S37),the temperature set value Tsv is then reduced by 0.05 (° C.) to bestored in the EEPROM 706 as an optimal temperature value (S38), therebyterminating the foregoing element temperature learning processes. Next,it is checked if a harmonic wave with an output power of 2.6 W isobtained (S39).

As a result of check in S39, if the output power is equal to or largerthan 2.6 W, the sequence is terminated via the S42 where it is checkedif the margin for the upper limit current is not more than 5% (S42).This is because the output power can be adjusted by reducing the inputpower (YES in S39 and NO in S42). On the other hand, if the output powerof the harmonic wave is smaller than 2.6 W (NO in S39 and NO in S40),the input power is increased by 10% to perform the temperature scanningagain (S41). If the target output power level of the harmonic wave stillcannot be obtained after performing the re-scanning of the temperature,an error flag is outputted (S43), and the driving of the device isstopped. On the other hand, if the target output power level of theharmonic wave is obtained after carrying out the re-scanning, it isdetermined if the margin left for the upper limit current is less than5% (S42). If it is determined that the margin left is less than 5%, anerror flag is outputted (S43).

Upon completing the processes for learning the holding temperature ofthe wavelength conversion element 209, the processes of controlling thewavelength conversion element 209 by the temperature controller 202 innormal state shown in the flowchart of FIG. 8 are to be executed.

FIG. 12 is a graph showing differences in conversion efficiency betweenthe case wherein temperature learning processes are executed with anoptimal holding temperature set for each output power level (firstembodiment) and the case wherein temperature learning processes areexecuted with an optimal holding temperature set for an upper limitoutput power level (present embodiment).

As can be seen from the graph of FIG. 12, when adopting the temperaturelearning processes in accordance with the present embodiment, the targetoutput power level of the harmonic wave was not able to be obtained withthe input power of the fundamental wave of less than 5 W. On the otherhand, when executing the temperature learning processes with an optimalholding temperature set for each output power level as in the firstembodiment, the harmonic wave of the output power of 1 W or higher isobtained with the input power of the fundamental wave of 5 W.

As explained, the element temperature learning method in accordance withthe present embodiment is disadvantageous in that the conversionefficiency is lowered as compared to the case of adopting the method ofthe first embodiment. On the other hand, the element temperaturelearning method is advantageous in that the time required for learningcan be reduced as compared to the case of adopting the method of thefirst embodiment wherein the element temperature is obtained for eachoutput power of the harmonic wave.

Additionally, the foregoing method of setting the element temperature inaccordance with the present embodiment is also advantageous in thefollowing point. That is, the output power of the second harmonic waveis saturated even when the input power of the fundamental wave isincreased. It is therefore possible to prevent the wavelength conversionelement from being damaged due to an increase in light absorption ratioin proportion to the power of the second harmonic wave generated byabsorbing the second harmonic wave, two-photon absorption in particular.

Third Embodiment

The following descriptions will explain still another embodiment of thepresent invention with reference to figures.

In the present embodiment, a peak search is to be executed for anoptimal temperature of the wavelength conversion element 209 for outputpowers set to some hundreds mW which is lower than that of the secondembodiment. By reducing the element temperature according to thefundamental wave as inputted, such problem can be avoided when carryingout the auto power control. That is, the output power cannot becontrolled with an increase in input fundamental wave.

The graph of FIG. 13 shows the relationship between the optimaltemperature of the wavelength conversion element (shift in temperaturefrom the phase matching temperature) and the second harmonic wave outputgenerated at the optimal element temperature with respect to thefundamental wave input.

Specifically, as shown in the graph of FIG. 13, the optimal elementtemperature (reference element temperature) is learned with afundamental wave with an input power of 2 W is set as the referencefundamental wave. A shift in temperature from the reference elementtemperature corresponding to the fundamental wave different from thereference fundamental wave is recorded in the EEPROM 706. In this way,it is possible to shift the element temperature by computing thenecessary element temperature from the fundamental wave input requiredfor obtaining the harmonic wave set in the output setting device 201.

FIG. 14 is a flowchart showing processes of controlling a wavelengthconversion element by a temperature controller in accordance with thepresent invention in normal state. FIG. 15 is a flowchart showingprocesses of controlling a wavelength conversion element by atemperature controller when executing the element temperature learningprocesses in accordance with the present embodiment.

In the present embodiment, as a preparation state for controlling theelement temperature in accordance with the present embodiment, anoptimal element temperature T is obtained when outputting lower powerharmonic wave from the EEPROM 706 (S51). Then, the temperature of theharmonic wave conversion element 209 is adjusted to the elementtemperature T, and the device is set in a standby state (S52).

Upon starting the processes of controlling the element temperature(553), the output set value Psv is obtained from the EEPROM 706 (S54).Next, based on the output set value Psv thus obtained, the input setvalue Isv and the ΔT required for the target output level are obtainedfrom the EEPROM 706 (S55).

Next, ΔT thus obtained in S55 is subtracted from the optimal elementtemperature T for low power output (S56), and the resulting temperatureis set (S57). The temperature learning processes of the presentembodiment differ in that only ΔT is used for the information indicativeof target temperature of the wavelength conversion element from otherembodiments which use information indicative of temperaturescorresponding to the output power levels of the harmonic wave.

Next, the temperature present value Tpv of the wavelength conversionelement 209 is obtained from the thermistor 703 (S58). Then, theprocesses for controlling current to be applied to the temperatureholder 216 are started with regard to the waveform and the polarity (inthe case of adopting Peltier device as the temperature holder 216) ofthe current to be applied to the temperature holder 216 in the samemanner as the first embodiment explained with reference to the flowchartof FIG. 8.

A coefficient G for use in controlling the current waveform is computedfrom the following formula (6) based on the temperature initial value T,the set temperature value Tsv, and the temperature present value Tpv(S59).

G=(Tpv−T)/(Tsv−T)  (6).

In the present embodiment, the current waveform is controlled byswitching a duty ratio, based on the coefficient G computed in S58.

When the condition of G≧0.9 is satisfied (YES in S60), the PWM controlis performed (S61). On the other hand, when the condition of G≧0.9 isnot satisfied (NO in S60), the duty ratio is set to 100%, and the PWMcontrol is not performed (S62).

In replace of the foregoing structure of switching the duty ratio basedon the coefficient G, it may be arranged so as to directly use thecomputed value from the equation of the current duty ratio forcontrolling temperature=b*(1−G)/(b+G) using the coefficient G and thecoefficient b (0<b≦1).

Next, it is determined if a change in current polarity is required bythe following inequality (7) using Tpv, Tsv and a coefficient a (S63).

Tpv>a×Tsv  (7).

When the above inequality (7) holds (YES in S63), the polarity ofcurrent is changed (S64). On the other hand, when the inequality (7)does not hold (NO in S63), it is determined if the above inequalityholds in S63 without performing the PWM control in S61.

The coefficient a in the inequality (7) is adopted for preventing thepolarity of current to be switched to the normal state frequently. It ispreferable that the coefficient a falls in a range of from 1.1 to 1.2.

As in the case of the first embodiment, when Tpv is in a range of 1.1Tsv to 1.2 Tsv, the current may be cut to reduce the temperature of thewavelength conversion element 209 by natural cooling.

After the polarity of current changes in S64, the sequence goes back toS58 where the current temperature value of the wavelength conversionelement 209 is obtained. By repeating the foregoing processes, a greenlight can be outputted under stable conditions.

Then, the temperature present value Tpv is obtained, and the processesfor controlling current to be applied to the temperature holder 216 arestarted with regard to waveform and polarity (in the case of adoptingPeltier device as the temperature holder 216) based on the temperatureinitial value T, the temperature set value Tsv, and the temperaturepresent value Tpv.

Here, the temperature set value is not changed unless the output powerset value changes via the control unit 225 from the output settingdevice 201. Namely, when the auto power control is performed, it isknown the fundamental wave input level is subjected to fluctuations. Inthe present embodiment, the temperature of the wavelength conversiontemperature is not adjusted for such fluctuations.

Next, the processes for learning temperature of the wavelengthconversion element 209 will be explained with reference to FIG. 15.

Upon starting the element temperature learning processes, first, theoutput set value Psv for learning is obtained from the EEPROM 706. Here,the output set value Psv for learning is set to be low. Specifically,the output level of the harmonic wave outputted from the wavelengthconversion element 209 is set in a range of from 100 mW to 200 mW. Bysetting the amount of the laser beam generated while the temperaturelearning processes are being carried out to be such a low level, it ispossible to prevent the light beam from being leaked out of the devicemain body through a shutter, for example. In the present embodiment, thetemperature set value Tsv is determined based on the output set valuePsv for learning.

Upon starting the element temperature learning processes, first, theinput current I_(p0) for learning is obtained from the EEPROM 706 (S71).In the meantime, the initial temperature (Tsv₀) for learning is obtainedfrom the EEPROM 706 (S72). For the element temperature learningprocesses of the wavelength conversion element 209, a phase matchingtemperature of the wavelength conversion element 209 is searched basedon peak hold by scanning temperatures. Therefore, the temperature setvalue is set to 0.5 (° C.) below Tsv (S73).

Then, the harmonic wave with an output level of Ppv₁ is actuallygenerated, to be stored in the register 705 (S74). The temperature setvalue Tsv is then increased by 0.05 (° C.) to be replaced with thecurrently set temperature (S75).

Next, the harmonic wave with an output level of Ppv_((n+1)) of the nthloop is obtained to be temporarily stored in the register 705 (S76). Theharmonic wave with an output level of Ppv_((n+1)) is then compared withthe harmonic wave with an output level of Ppv_(n) (S77).

Ppv _(n) >Ppv _((n+1))  (8)

When the above inequality (8) does not hold (NO in S77), the temperatureset value Tsv is increased again by 0.05 (° C.) to be replaced with thecurrently set temperature repetitively. On the other hand, if the aboveinequality (8) holds (YES in S77), the sequence goes out of the loopand, the temperature set value Tsv is then reduced by 0.05 (° C.) to bestored in the EEPROM 706 as an optimal temperature value (S78), therebyterminating the foregoing element temperature learning processes.

In the foregoing element temperature leaning processes, the initialtemperature set value is set to 0.5 (° C.) below Tsv. However, how manydegrees to be decreased from the Tsv is to be determined based on thetolerance range for temperatures (temperature range at which the outputlevel is ½) of the wavelength conversion element 209, i.e., half widthat half maximum, HWHM from the peak to the ½ of the output level.

Here, in order to prevent the misreading of the sub-peak in thetemperature tuning curve of the harmonic wave output as the peak, it isdesirable that a drop in temperature be set to 0.5 to 1 times of thetolerance range for the temperature Δt of the wavelength conversionelement 209.

As described, the foregoing present embodiment performs the sameoperation as those of the first and second embodiments except for thatthe input level for fundamental wave for learning is set for a smalloutput level for the harmonic wave as compared to those in the first andsecond embodiments.

Fourth Embodiment

The following descriptions will explain still another embodiment of thepresent invention with reference to figures.

In the present embodiment, the method of determining a re-setting timingfor the holding temperature and the method of making fine adjustments onthe temperature of the wavelength conversion element 209 will beexplained in consideration of changes in desirable holding temperatureof the wavelength conversion element 209 as time passes.

As to the method of determining the timing for executing the elementtemperature learning processes mentioned in the first embodiment, thepresent embodiment adopts the following method. That is, the timing forexecuting the element temperature learning processes is determined basedon how closer to the upper limit current value for the pump LD 232, isthe current value required to be applied to the pump LD 232 in order toobtain the harmonic wave of an output level as set in the output settingdevice 201.

Firstly, the ratio of the current value I_(pv) in the pump LD 232 to thecurrent limit current value I_(lim) to the pump LD 232 is monitored, todetermine the re-setting timing for the temperature of the wavelengthconversion element 209 as explained below.

In this method, in the control loop for carrying out the auto powercontrol of the harmonic wave output, the process of computing theI_(pv)/I_(lim) is incorporated, so that the ratio of the current valueIpv that flows in the pump LD 232 to the upper limit current value forthe pump LD 232 can be monitored through out the element learningprocesses.

In the present embodiment, the ratio of the current value I_(pv) to thecurrent limit value I_(lim) of the pump LD 232 satisfies the followinginequality (9), it is determined that the ratio falls in a normal rangeto perform the normal state operation.

0<I _(pv) /I _(lim)<0.8  (9).

On the other hand, the ratio of the current value I_(pv) to the currentlimit value I_(lim) of the pump LD 232 satisfies the followinginequality (10), it is determined that the ratio falls in an abnormalrange, and therefore determined that the element temperature learningprocesses are to be executed.

0.8≦I _(pv) /I _(lim)  (10).

Here, an auto power control method may be adopted, wherein the currentvalue required to be applied to the pump LD 232 to obtain the harmonicwave of the output level as set in the output setting device is recordedin the EEPROM 706 before or after driving.

Specifically, it may be arranged so as to execute the elementtemperature learning processes when the following equation is to besatisfied:

I ₁×(1+d/100)=I _(lim)  (11)

wherein I₁ indicates a required LD current value for obtaining theoutput P₁, and d (%) indicates a margin required for the limit valueI_(lim) of the current to the pump LD 232 and the auto power control.

Here, a program may be set to instruct a computer to automatically carryout the temperature learning processes when determined based on theequation (11) that it is desirable to execute the element temperaturelearning processes while informing the user of that the device is beingin the element temperature learning processes. Alternatively, it may bearranged such that the user operates the device to execute thetemperature learning processes in response to a caution to do so.

In addition to the method of determining the timing for executing theelement temperature learning processes based on how closer to the upperlimit current value of the current to be applied to the pump LD 232, isthe current value required for the pump LD 232 to obtain the harmonicwave of an output level as set in the output setting device 201, thefollowing method will be explained.

That is, a harmonic wave output is generated using a rectangularmodulation, and the modulation waveform of the driving current iscompared with the waveform of the harmonic wave output, and the presenttemperature of the wavelength conversion element 209 is determined andis adjusted (fine adjustment) based on which part of the output waveformis lacked.

As another method of determining the timing for executing the elementtemperature learning processes, a method of making fine adjustments onthe temperature of the wavelength conversion element will be explained,wherein a harmonic wave output is generated using a rectangularmodulation, and the waveform of the current to be inputted to thefundamental wave light source 231 is compared with the waveform of theharmonic wave output.

FIGS. 16A to 16C show the relationship between the temperature of thewavelength conversion element 209 and the harmonic wave output pulsewaveform. FIGS. 16A to 16C show the harmonic wave output waveform whenapplying the current pulse waveform with the pulse width of 20 msec andthe duty of 33% as a modulation signal to the light source in the caseof obtaining the harmonic wave of the output level of 2.2 W in responseto the fundamental wave input of 6.5 W at a wave height value. FIG. 16Ashows the case where the temperature of the wavelength conversionelement 209 is 0.3 (° C.) below the phase matching temperature; FIG. 16Bshows the case where the temperature of the wavelength conversionelement 209 is at the phase matching temperature; and FIG. 16C shows thewhere the temperature of the wavelength conversion element 209 is 0.3 (°C.) above the phase matching temperature. Incidentally, the dotted lineshown in FIG. 16A and FIG. 16C show the shape of the current signalwaveform to be inputted to the fundamental wave light source.

As shown in FIG. 16A, when the temperature of the wavelength conversionelement 209 is 0.3 (° C.) below the phase matching temperature, theelement temperature is low, and the harmonic wave is at the low level atthe moment the current pulse is inputted. However, the temperature ofthe wavelength conversion element 209 is raised by the harmonic wave asgenerated, and the wavelength conversion element 209 becomes in thephase matching state. Therefore, in the case of FIG. 16A, the waveheight value as set can be obtained for the latter half of the outputwaveform as modulated, and the output waveform is lacked in a range of20% to 50% at the beginning (at a rise) of the output waveform.

On the other hand, as shown in FIG. 16C, when the temperature of thewavelength conversion element 209 is 0.3 (° C.) above the phase matchingtemperature, the temperature of the wavelength conversion element 209 isat around the phase matching temperature and the harmonic wave of thewave height value as set can be obtained at the moment the current pulseis inputted. However, as the temperature of the wavelength conversionelement 209 is further raised by the harmonic wave as generated, thewavelength conversion element 209 becomes out of the phase matchingstate. Therefore, in the case of FIG. 16A, in the latter half of theoutput waveform as modulated, the wave height value as set cannot beobtained, and the output waveform is lacked in a range of 20% to 50% atthe end (at a fall) of the output waveform.

As shown in FIG. 16B, in the phase matching state, the current pulsewaveform is matched with the waveform of the harmonic wave actuallyoutputted. As described, by comparing the current pulse waveform withthe waveform of the harmonic wave output, it can be observed to whichdirection (lower temperature side or higher temperature side), thetemperature of the wavelength conversion element 209 is shifted from thephase matching temperature.

FIG. 17 is a flowchart showing processes of making fine adjustment intemperatures of the wavelength conversion element 209 based on pulsewaveform in accordance with the present embodiment adopting theforegoing method.

Firstly, processes in the fine adjustment modes are executed in thestate where the harmonic wave output is in the rectangular modulation.Here, it may be arranged so as to set the light emission mode to theoutput modulation (rectangular modulation) when activating the fineadjustment mode.

As shown in the flowchart of FIG. 17, first, the waveform of the currentto be inputted to the fundamental wave light source 231 is compared withthe waveform of the harmonic wave (S81). As a result of comparison inS81, if the current waveform does not match the harmonic wave waveform(NO in S81), it is judged if the temperature of the wavelengthconversion element 209 is shifted to the lower temperature side from thephase matching state (S82), i.e., if the harmonic wave waveform islacked at the beginning (at a rise) as shown in FIG. 16A.

If it is judged that the temperature of the wavelength conversionelement 209 is shifted to the lower temperature side from the phasematching state (YES in S82), the temperature of the wavelengthconversion element 209 is increased until the current waveform ismatched with the harmonic wave waveform (YES in S82) by 0.05 (° C.) eachtime (S83).

On the other hand, if it is judged that the temperature of thewavelength conversion element 209 is shifted to the higher temperatureside from the phase matching state (NO in S82), the temperature of thewavelength conversion element 209 is decreased until the currentwaveform is matched with the harmonic wave waveform (YES in S82) by 0.05(° C.) each time (S84).

With the foregoing processes, when the current waveform is matched withthe waveform of the harmonic wave output, fine adjustment processes areterminated (YES in S82). Then, the new element temperature set value isrecorded in the EEPROM 706 (S85).

Here, it is desirable that the temperature of the wavelength conversionelement 209 is increased in a range of 0.05 (° C.) to 0.1 (° C.) eachtime. The lower limit of 0.05 (° C.) is set in consideration of thetemperature detection precision, and the upper limit of 0.1 (° C.) isset in consideration of suppressing fluctuations in output level. Thetemperature of the wavelength conversion element 209 can be maintainedin an appropriate temperature range by the foregoing processes, and theprocesses of making fine adjustments are terminated when the waveform ofthe current pulse is matched with the waveform of the harmonic wave.

For example, when warming up after turning ON the power supply, thecurrent pulse waveform with the pulse width of 20 msec and the duty of33% is inputted as a modulation signal to the fundamental wave lightsource 231, and temperature of the wavelength conversion element 209 isadjusted to a phase matching temperature, based on the waveform of theharmonic wave as observed. Alternatively, the temperature of thewavelength conversion element 209 may be adjusted to the phase matchingstate based on the waveform of the harmonic wave output as observed whenactually using in the output modulation state. This method isadvantageous in that the temperature of the wavelength conversionelement 209 can be adjusted while the device is being used.

Incidentally, in consideration of the feedback to the elementtemperature based on the waveform shape of the modulated harmonic wave,it is desirable that the modulation signal has a pulse width of not lessthan 100 μsec.

Alternatively, for example, a Q switch may be used for the fundamentalwave light source 231 to oscillate a high peak pulse. In this case, thetemperature of the wavelength conversion element 209 can be set byobtaining the pulse string of the harmonic wave output of not less than100 μsec, and comparing the envelope with the current signal waveform.

Fifth Embodiment

The following descriptions will explain still another embodiment of thepresent invention with reference to figures.

A wavelength conversion laser light source 200 in accordance with thepresent embodiment is arranged so as to change a beam diameter of thesecond harmonic wave itself according to the intensity of the secondharmonic wave 220 by the wavelength conversion element (nonlinearoptical crystals) 209 according to the intensity of the second harmonicwave 220.

To realize the foregoing structure, the photoreceptor 212 of thewavelength conversion laser light source 200 restricts a value of theharmonic wave output based on changes in beam diameter.

In the present embodiment, MgO: LiNbO₃ crystal element is adopted as thenon-linear optical crystals, wherein the periodical polarizationinversion structure is formed. By using the periodical polarizationinversion structure MgO: LiNbO₃ crystal element, the conversionefficiency from the fundamental wave to a green light (the secondharmonic wave) can be improved significantly. Furthermore, when adoptingthe crystal element in the wavelength conversion optical system whichsatisfies the confocal condition, it is possible to realize the functionof varying the beam diameter according to the intensity of the secondharmonic wave output.

When adopting the MgO: LiNbO₃ crystal element having the periodicallypoled structure as non-linear optical crystals as in the presentembodiment, it becomes clear that the beam shape changes according to anoutput level of the second harmonic wave generated by the wavelengthconversion.

An example of the optical system, wherein a beam shape varies accordingto an output level of a second harmonic wave, will be explained throughan example shown in FIG. 18A. The graph of FIG. 18A shows therelationship between the second harmonic wave output level (wavelengthof 532 nm) with respect to the fundamental wave (wavelength of 1064 nm).

As shown in the graph of FIG. 18A, it is observed that the beam diameteris stabilized in a range of the output of the second harmonic wave from0 W to 1.6 W, and hardly varies (FIG. 18B). On the other hand, from thepoint where the output of the second harmonic wave becomes larger than1.6 W, it is observed that the beam diameter gradually becomes smaller,and at a point the output of the second harmonic wave exceeds 3 W, thebeam becomes in a doughnut shape, and the harmonic wave output becomesunstable (FIG. 18C).

When adopting the MgO: LiNbO₃ crystal element (MgLNO₃ element), it isconfirmed that for the second harmonic wave output range (530 nm) in arange of from 2.3 W to 3.0 W, the fundamental wave input is not largerthan 6.5 w, and an abnormality in beam shape is not observed. In thecase of adopting LiTaO₃ of stoichiometry having added thereto MgO, evenfor the output of the second harmonic wave of up to 9 W, the beam doesnot become in a doughnut shape. In view of the foregoing, it isconfirmed that the output of the second harmonic wave, with which anabnormality in beam shape is observed, changes according to thewavelength. Specifically, it is confirmed that the longer is thewavelength, the larger is the output level.

The second harmonic wave 220 having wavelength converted by thewavelength conversion element (non-linear optical crystal) 209 is formedinto a parallel beam by the re-collimating lens 211. After having formedinto the parallel beam, the second harmonic wave 220 is separated by thebeamsplitter 213, and is partially received by the photoreceptor(photodiode) 212. On the other hand, a main beam of the second harmonicwave 220 passes through the beamsplitter 213, and is optically connectedto a delivery optical fiber 206 from the optical fiber incident surface206 a by the coupled lens 214. Here, the beam diameter of the secondharmonic wave 220 changes, which in turn changes the NA at the focalposition 701 of the beam as converged by the coupled lens 214.

For the detection of an abnormality in beam diameter, the photoreceptor212 is divided into the first region 1901 and the second region 1902. Inthe present embodiment, it is arranged such that a normal beam 1903 isincident in the first region 1901.

According to the foregoing structure, when the output power becomes toolarge, and the output is formed in a doughnut beam 1904, the light beamis incident in the second region 1902 of the photoreceptor 212.

FIG. 20A is a block diagram schematically showing a structure of acontrol unit of a mechanism of detecting an abnormality in beam diameterin accordance with the present embodiment. FIG. 20B is a flowchartshowing control processes by the control unit 225 for detecting anabnormality in beam diameter in accordance with the present embodiment.

As shown in FIG. 20A, the control unit 225 includes, an A/D converter2001, a D/A converter 2002, an MPU 2003, and a register 2004.

Next, the control processes for detecting an abnormality in beamdiameter will be explained in reference to the flowchart of FIG. 20B. Anamount of light received is obtained in the first region 1901 of thephotoreceptor 212, and is stored (S91). An amount of light received isobtained also in the second region 1902 of the photoreceptor 212, and isstored (S92). These amounts of light received are converted into digitalsignals by the A/D converter 2001, and are then stored in the register2004 shown in FIG. 20A. The MPU 2003 computes a difference usingrespective amounts of light as stored (S93).

Here, the current to be applied to the pump LD 232 is reduced (S95) whenthe following inequality is satisfied (YES in S94) to avoid anabnormality in beam shape.

d×P1901<P1902  (12)

wherein P1901 indicates an amount of light received in the first regionP1901, and the second region P1902 indicates an amount of light receivedin the second region 1902. In this state, the information indicative ofa low power mode is displayed in, for example, a consol of the device(S96). In the present embodiment, the coefficient d adopted in thedifference computation may be set to 0.5, for example.

Then, amounts of light as received respectively in the first region 1901and the second region 1902 are obtained again (S97 and S98), and adifference computation is performed (S99). As a result, if the beam in adoughnut shape is not observed (NO in S100), the driving is continued inlow power mode. On the other hand, if the beam in a doughnut shape isstill observed (YES in S100) for the second time difference computation,the system is reset, and an error signal is generated (S101). Here, thesteps of resetting the system and generating an error signal areperformed based on the second time difference computation. However, thepresent embodiment is not intended to be limited to the foregoingmethod, and, for example, the steps of resetting the system andgenerating an error signal may be performed based on the third timedifference computation.

FIG. 21A through FIG. 21C are explanatory views, which depict examplesof a photoreceptor of a mechanism of detecting an abnormality in beamdiameter in accordance with the present embodiment.

The difference computation is performed with a coefficient d of 0.5 inthe present embodiment; however, the coefficient d can be set in a rangeof from 0 to 0.5 in consideration of a stray light generated in theoptical arrangement, or the like.

For the photoreceptor 212 (photodiode, etc.), the present embodiment isnot intended to be limited to those divided into two regions, and, forexample, those divided into three regions (regions 2101, 2102 and 2103)as shown in FIG. 21A, and those divided into four regions may beadopted. As shown in FIG. 21B, other than above examples, aphotoreceptor 212 masked with a pinhole 2104 as shown in FIG. 21B, orthe CCD (Charge Coupled Device) element 2105 may be used as shown inFIG. 21C.

The photoreceptor 212 masked with the pinhole 2104 shown in FIG. 21B ismost suited in terms of manufacturing costs; however, when adopting thisphotoreceptor 212, it is required to determine if an abnormality in beamshape occurs by collating with the input value of the current applied tothe pump LD 232. In the case of adopting the CCD element 2105 shown inFIG. 21C, it is required to perform the image processing, the imageprocessing, or the image process when overflowing charges are required.The divided photodiode shown in FIG. 19B or FIG. 21A is the mostdesirable for convenience in use.

As the method of preventing an abnormality in beam diameter, the methodof shifting the focal position of the fundamental wave may be adopted,which offers the same effect.

FIG. 22A is an explanatory view schematically showing the structure ofthe control unit 225 for detecting the abnormality in beam diameter inaccordance with the present embodiment. FIG. 22B is a flowchart showingcontrol processes by the control unit 225 for detecting an abnormalityin beam diameter in accordance with the present embodiment.

The second harmonic wave 220 having wavelength converted by thewavelength conversion element (non-linear optical crystal) 209 is formedinto a parallel beam by the re-collimating lens 211. After having formedinto the parallel beam, the beam is separated by the beamsplitter 213into the harmonic wave by the photoreceptor (photodiode) 212 and theother beam. On the other hand, a main beam of the second harmonic wave220 passes through the beamsplitter 213, and is outputted to theoutside.

For the detection of an abnormality in beam diameter of the secondharmonic wave 220, adopted is the photoreceptor 212 divided into thefirst region 1901 and the second region 1902, to obtain respective lightamounts received in the first region 1901 and the second region 1902,and a difference computation of light amounts respectively received inthese regions is performed. As a result of difference computation, if itis judged that the beam is in a doughnut shape, the collective lens 208is shifted from the position indicated by 208(a) to the positionindicated by 208(b) so as to shift the focal position 2201 of thefundamental wave 235 to the light incident side (to the side of thefocal position 2202). In this way, the beam in a doughnut shape becomesno longer observed; however, the amount of the second harmonic wave 220generated would be reduced. In response, the current applied to the pumpLD 232 is increased, to maintain the light amount so that the outputlevel of the second harmonic wave can be maintained while eliminatingthe beam in doughnut shape.

Next, the control processes for detecting an abnormality in beamdiameter will be explained in reference to the flowchart of FIG. 22B.The respective amounts of light received in the first region 1901 andthe second region 1902 of the photoreceptor 212 are converted intodigital signals by the A/D converter 2001, and are then stored in theregister 2004 (S111 and 5112). The MPU 2003 computes a difference usingrespective amounts of light as stored (S113).

Here, the collective lens 208 is shifted by a predetermined distance(S115) to shift the focal position of the fundamental wave 235 to thelight incident side when the inequality (13) is satisfied (YES in 5114)to avoid an abnormality in beam shape.

d×P1901<P1902  (13)

wherein P1901 indicates an amount of light received in the first regionP1901, and the second region P1902 indicates an amount of light receivedin the second region 1902.

Then, amounts of light as received respectively in the first region 1901and the second region 1902 are obtained again (S116 and S117), and adifference computation is performed (S118). As a result, if the beam ina doughnut shape is still observed (YES in S119), the sequence goes to5115 where the collective lens 208 is shifted again by the predetermineddistance.

On the other hand, if the doughnut beam is not observed (NO in 5119),the amount of light of the second harmonic wave incident in the firstregion 1901 is checked (S120). If the amount of light of the secondharmonic wave is not sufficient (NO in 5120), the current applied to thepump LD 232 is increased by a predetermined amount (S121). Then, thedriving is continued while repeating the foregoing steps 116 to S121until the amount of light of the second harmonic wave reaches the setvalue (YES in S120).

As described, the amount of light of the fundamental wave is increasedby increasing the current applied to the pump LD 232, and in themeantime, an amount of the second harmonic wave incident in the firstregion 1901 is checked. Then, the processes are carried out along theflowchart of FIG. 22B, to maintain the second harmonic wave at the setoutput level while eliminating the beam in an abnormality shape(doughnut shape).

In the present embodiment, a green laser to be incident into the fiber,with the beam quality (M² value) of 1.3 is adopted; however, it isconfirmed that the sufficient effect as achieved from the presentinvention can be achieved as long as the green laser with the beamquality (M² value) of not higher than 2 is adopted.

In the foregoing first through fifth embodiments, for the non-linearoptical crystals, crystals which absorb light (two-photon absorption,for example) by a harmonic wave output, i.e., the wavelength of theharmonic wave generated, it is required to be less than 2 times of awavelength of an absorption edge of nonlinear optical crystals of thewavelength conversion element. Examples of the nonlinear opticalcrystals of the wavelength conversion element satisfying the abovecondition includes: lithium niobate (including those having magnesiumoxide added), lithium tantalite (including those having magnesium oxideadded), KTiOPO₄ (KTP: Potassium Titanyl Phosphate), and the wavelengthconversion element generates a green light having a wavelength in arange of from 520 nm to 560 nm from Nd: YAG laser or Yb fiber usingthese crystals.

As can be seen also from the graph of FIG. 3 which shows therelationship between the harmonic wave output and the fundamental waveinput, when obtaining the output level of 500 mW using the wavelengthconversion element whose light absorption ratio increases in proportionto the second power of the second harmonic wave generated by absorbingthe second harmonic wave, two-photon absorption in particular, effectsas achieved from the element temperature setting method in accordancewith present embodiment can be appreciated.

As described, according to the foregoing method of the presentembodiment, the wavelength conversion element having the light absorbingproperties induced by the harmonic wave is used, and the temperature ofthe wavelength conversion element is set to a temperature at which theharmonic wave output level of 120% of that actually used can beobtained. With this method of searching the phase matching temperature,i.e., an optimal temperature at which the wavelength conversionefficiency can be maximized, even if the fundamental wave input levelbecomes too high, the output of the second harmonic wave would besaturated. It is therefore possible to prevent the wavelength conversionelement from being damaged due to an increase in light absorption ratioin proportion to the second power of the second harmonic wave generatedby absorbing the second harmonic wave, two-photon absorption inparticular as occurred when searching for a phase matching temperatureas an optimal temperature at which the highest wavelength conversionefficiency can be ensured.

FIG. 23, FIGS. 24A and 24B show an example structure of atwo-dimensional image display apparatus adopting the fiber laser lightsource in accordance with the present embodiment.

One example structure of a laser display (image display apparatus)applied to the wavelength conversion module of the present embodimentwill be explained in reference to FIG. 23.

FIG. 23 schematically shows the structure of an optical engine of aprojector system adopting a laser light source in accordance with thepresent embodiment.

The two-dimensional image display device 2300 in accordance with thepresent embodiment has an optical engine for a projector using 3 LCDpanels. The two-dimensional image display device 2300 includes an imageprocessing section 2302, a laser output controller (controller) 2303, anLD power supply 2304, red, green and blue laser light sources 2305R,2305G, and 2305B, beam shape rod lens 2306R, 2306G, and 2306B, relaylens 2307R, 2307G and 2307B, reflecting mirrors 2308G and 2308B,two-dimensional modulation elements 2309R, 2309G and 2309B fordisplaying an image; polarizers 2310R, 2310G and 2310B, a combine prism2311, and a projection lens 2312.

The green laser light source 2305G is controlled by the controller 2303and the LD power supply 2304 which control an output from the greenlight source.

A laser beam emitted from each of the red, green and blue laser lightsources 2305R, 2305G, and 2305B are formed in a rectangular shape by thebeam shape rod lens 2306R, 2306G, and 2306B, and with which, thetwo-dimensional modulation element in each color is illuminated by therelay lens 2307R, 1307G, and 2307B. Further, two-dimensionally modulatedimages in respective colors are combined by the combine prism 2311, andthe resulting image is projected onto the screen by the projection lens2312, thereby displaying an image.

The green laser light source 2305G is arranged such that a laserresonator is housed in the fiber. With this structure, it is possible tosuppress a reduction in output level and fluctuations in output power astime passes due to an increase in loss in the resonator by dustparticles from the outside or a misalignment of the reflective surface.

On the other hand, in the image processing section 2302, a light amountcontrol signal is generated for changing the output level of the laserbeam according to the luminance information of the input image signal2301, and transmits the light amount control signal to the laser outputcontroller 2303. According to the foregoing image processing section2302, a contrast can be improved by controlling the light amountaccording to the luminance information.

In this case, the control method (PWM control) may be adopted, whereinan average light amount is changed by pulse driving the laser beam tochange the duty ratio (ON time)/(ON time+OFF time) for the ON time ofthe laser.

The green light source adopted in this projection system may be arrangedso as to emit a Green laser beam having a wavelength in a range of from510 nm to 550 nm. With this structure, it is possible to obtain anoutput laser beam in Green color of high spectral luminous, therebyrealizing a display with a desirable color reproducibility, which candisplay an image in color close to an original color.

Specifically, the two-dimensional image display device of the presentinvention includes a screen, a plurality of laser light sources, ascanning section for scanning the laser light sources, wherein the laserlight sources include at least laser sources which emit a red colorlaser beam, a green color laser beam, and a blue color laser beam; andat least the green color light source is provided with the wavelengthconversion element having any of the foregoing structures.

With the foregoing structure, an output laser beam in Green color ofhigh spectral luminous can be obtained. It is therefore possible torealize a color still closer to the original color with an applicationto a display with a desirable color reproducibility.

For the two-dimensional image display device, those of a rear projectiondisplay type (FIG. 23), or of a front projection type may be adopted.

For the special modulation element, it is needless to mention that atwo-dimensional modulation element of the transmission type liquidcrystal or the reflective type liquid crystal, a galvanometer mirror, aDMD or other Micro Electro Mechanical System (MEMS) may be used.

When adopting the light modulation element which is less likely to beaffected by polarization components with respect to the light modulationcharacteristics, such as the reflection-type special modulation element,the MEMS, the galvanometer mirror like the case of the presentembodiment, it is not required to adopt a polarization-maintainingoptical fiber such as a PANDA (polarization maintaining and absorptionreducing) fiber for transmitting the harmonic wave with the opticalfiber. On the other hand, when adopting a two-dimensional modulationdevice using liquid crystals, the modulation property and thepolarization property are significantly affected. It is thereforedesirable to adopt a polarization-maintaining optical fiber.

FIG. 24 shows one example structure of a display adopting the laserlight source. A liquid crystal display 2400 includes, for example, alaser light source 2402, a control unit 2403, a light guide member 2404for converting the laser light source from a point light source to alinear light source, a light guide plate 2408 for converting the linearlight source into a planar light source to be projected onto the entireliquid crystal panel, a polarization plate/diffusion member 2409 foraligning the polarization direction or removing the non-uniformillumination, and a liquid crystal panel 2410, etc. Namely, the lightsource of the present invention may be used as a backlight source forthe liquid crystal display.

As shown FIG. 25, the laser device of the present invention may be usedas a laser light source 2500 for surgical operations, which is made upof, for example, a laser light source, a control unit for controllingthe output from the laser light source, an output setting device 2502for setting an output level, an output connector 2503 for outputting alaser from a laser light source, a delivery fiber 205 for guiding thelaser beam to a desired area to be irradiated with, and a hand peace2505, etc.

Using the small beam diameter region with an output range of 1.6 W to 3W as shown in the graph of FIG. 18A, it is possible to excite the secondharmonic wave by optically connecting to the delivery fiber 2504 bychanging the NA according to needs. Specifically, in the case where onlythe output of low level is required, but the spatial uniformity of thelaser beam 2506 as emitted from the delivery fiber 2504 is required, theNA is set to be large (NA: 0.12 in the case of the present embodiment).On the other hand, in the case where the output of high level isrequired, but the uniformity is not required, the NA is set to be small(NA: 0.09 in the case of the present embodiment).

FIG. 26A is a graph showing the relationship between the light intensityand the beam diameter directly before being connected to atransmission-use fiber of the laser light source with fiber as shown inFIG. 25. FIG. 26B is a graph showing the relationship between a lightintensity and a transmission-use fiber NA of a laser beam to be incidentinto the delivery fiber 2504. As can be seen from the graphs of FIG. 26Aand FIG. 26B, the NA to the delivery fiber 2504 becomes smaller as thebeam diameter becomes smaller according to the output level.

According to the foregoing structures of the present invention, thefollowing problems when adopting the wavelength conversion elementhaving the light absorption properties can be prevented. That is, theauto power control: APC cannot be performed as the output level is notraised in response to an increase in input power of the fundamentalwave, or it becomes not possible to adjust changes in phase matchingtemperature of the wavelength conversion element as time passes. Asdescribed, since such problem that the output level becomes out ofcontrol can be prevented, an improved reliability of the apparatus canbe ensured, thereby realizing the wavelength conversion laser lightsource for displays suited for consumer products, etc.

As described, a wavelength conversion laser light source, according toone aspect of the present invention includes: a fundamental wave laserlight source; a wavelength conversion element for converting afundamental wave emitted from the fundamental wave laser light sourceinto a harmonic wave, the wavelength conversion element being made of amaterial whose light absorption properties change according to an outputlevel of a harmonic wave; an output setting section for setting aharmonic wave output power level; and an element temperature switchingsection that switches a temperature of the wavelength conversion elementaccording to a harmonic wave output level as set in the output settingdevice, wherein the element temperature switch section includes anelement temperature holding section which holds the wavelengthconversion element at the temperature as switched by the elementtemperature switching section.

According to the foregoing structure wherein the wavelength conversionelement made of a material whose light absorption properties changeaccording to an output level of a harmonic wave, the phase matchingtemperature of the wavelength conversion element changes according to anoutput level of the harmonic wave. In response to changes in phasematching temperature of the wavelength conversion element according tothe output level of the harmonic wave, the element temperature switchingsection switches the element temperature of the wavelength conversionelement according to the output level of the harmonic wave, and theelement temperature holding section holds the wavelength conversionelement at the element temperature as switched. As a result, it ispossible to realize the wavelength conversion laser light source, whichpermits an efficient conversion without being adversely affected bychanges in phase matching condition of the wavelength conversion elementaccording to the output level of the harmonic wave.

Furthermore, the wavelength conversion element can be maintained at adesirable temperature corresponding to the output level of the harmonicwave, thereby realizing a high wavelength conversion efficiently.

With the foregoing structure, it is preferable that the elementtemperature holding section includes a memory for storing thereinelement temperatures to be held by the element temperature holdingsection.

With the foregoing structure, it is preferable that the elementtemperature holding section includes a temperature adjusting membercapable of adjusting the temperature of the wavelength conversionelement by heating or cooling the wavelength conversion element, anelement temperature detector which detects the temperature of thewavelength conversion element, and a temperature controller whichcontrols the temperature of the wavelength conversion element bycontrolling the temperature adjusting member, wherein the temperaturecontroller controls the temperature adjusting member, to set a presenttemperature of the element temperature detector to the temperature asswitched by the element section.

According to the foregoing structure, since the temperature controllercontrols the temperature adjusting member based on the elementtemperature detected by the element temperature detector, the wavelengthconversion element can be maintained at a desirable temperatureaccording to the output level of the harmonic wave.

With the foregoing structure, it is preferable that the memory stores adriving current for the harmonic wave laser light source correspondingto a harmonic wave output power level.

According to the foregoing structure, the harmonic wave of the desiredoutput level can be obtained with ease based on the driving current forthe fundamental wave laser light source stored in the memory.

With the foregoing structure, it is preferable that an elementtemperature learning section be further provided for learning atemperature of the wavelength conversion element as stored in thememory, to rewrite the currently stored temperature into a temperatureas learned, wherein the element temperature learning section searchesfor an optimal element temperature at which a harmonic wave of thehighest power can be outputted, with a start temperature for the searchof a predetermined temperature Δt (° C.) below the temperature of thewavelength conversion element as stored in the memory, and the elementtemperature learning section carries out the search by increasing thetemperature from the start temperature, and stores in the memory theoptimal temperature as searched.

According to the foregoing structure, the element temperature learningsection carries out the learning by a peak search for a maximum harmonicwave output value while changing the temperature of the wavelengthconversion element. The element temperature learning section thendetermines a temperature, at which a harmonic wave of the highest powercan be outputted, as an optimal temperature corresponding to theharmonic wave output value. With this structure, even when the optimaltemperature of the wavelength conversion element corresponding to theharmonic wave output value changes from that stored in the memory due tothe deterioration of the wavelength conversion element as time passes,an optimal temperature obtained by carrying out the learning can bestored to be replaced with the currently stored temperature. As aresult, it is possible to perform the frequency conversion at highefficiency over a long period of time.

With the foregoing structure, it is preferable that an elementtemperature learning section be provided for learning a temperature ofthe wavelength conversion element as stored in the memory, to rewritethe currently stored temperature into a temperature as learned, whereinthe element temperature learning section searches for an optimal elementtemperature at which a harmonic wave of the highest power can beoutputted, with a start temperature for the search of a predeterminedtemperature Δt (° C.) below the temperature of the wavelength conversionelement as stored in the memory, and the element temperature learningsection carries out the search by increasing the temperature from thestart temperature, and stores in the memory a temperature lower than theoptimal temperature as searched.

According to the foregoing structure, as a result of learning, the lowertemperature than that obtained by the learning processes is stored inthe memory to be replaced with the currently stored temperature. It istherefore possible to prevent such problem that the output level of theharmonic wave cannot be increased with an increase in the input level ofthe fundamental wave. This can be seen from the tuning curve in thegraph of FIG. 4, which is shifted to the lower temperature side withrespect to the fundamental wave input.

With the foregoing structure, it is preferable that the predeterminedtemperature Δt (° C.) is in a range of 0.5 to 1 times of a tolerancerange for a temperature ΔT (° C.) of the wavelength conversion element.

According to the foregoing structure, when carrying out the peak searchfor the highest output level of the harmonic wave while changing thetemperature of the wavelength conversion element, a misreading of thesub-peak in the temperature tuning curve of the harmonic wave output asthe peak can be prevented.

With the foregoing structure, it is preferable that the elementtemperature learning section includes: a current monitoring sectionwhich monitors a driving current value of the fundamental wave laserlight source; and a learning timing determining section which determinesa start timing of the temperature learning processes; the learningtiming determining section determines that the temperature learningprocesses are to be started when the following equation holds:

I ₁×(1+d/100)=I _(lim)

wherein I₁ indicates a driving current value of the fundamental wavelaser light source required for obtaining a harmonic power of an outputpower value set in the output setting device, I_(lim) indicates adriving current limit value for the fundamental wave laser light source,and d (%) indicates a coefficient.

According to the foregoing structure, it is possible to determine anappropriate timing for executing the temperature learning processesbased on that the driving current value of the fundamental wave laserlight source becomes closer to the limit value of the driving currentfor the fundamental wave laser light source.

With the foregoing structure, it is preferable that the memory stores areference element temperature when a reference fundamental wave emittedfrom the fundamental wave laser light source is injected, and an amountof shift in temperature from the reference element temperaturecorresponding to the fundamental wave that is different from thereference fundamental wave, and the element temperature switch sectionswitches a temperature of the wavelength conversion element to atemperature as computed based on a shift in temperature corresponding tothe required fundamental wave for obtaining the harmonic wave of theoutput level set in the output setting section.

According to the foregoing structure, it is possible to shift thetemperature of the wavelength conversion element to an appropriatetemperature by computing the necessary element temperature for obtainingthe harmonic wave of the output level as set in the output settingdevice based on a shift in temperature from the reference elementtemperature.

With the foregoing structure, it is preferable that an elementtemperature fine adjustment section be further provided that makes afine adjustment on the temperature of the wavelength conversion elementin a state where a rectangular pulse beam is outputted from thewavelength laser light source, wherein the element temperature fineadjustment section adjusts the element temperature so as to match awaveform of a current pulse for use in driving the fundamental wavelight source with a waveform of the harmonic wave output as a result ofcomparison between these waveforms.

According to the foregoing structure, a rectangular pulse is outputtedfrom the wavelength conversion laser light source. Here, as shown inFIG. 16A, when the temperature of the wavelength conversion element islower than the phase matching temperature, the output waveform is lackedat the beginning (at a rise) of the output waveform of the harmonicwave.

On the other hand, as shown in FIG. 16C, when the temperature of thewavelength conversion element is higher than the phase matchingtemperature, the output waveform is lacked at the end (at a fall) of theoutput waveform of the harmonic wave. In the phase matching state, thecurrent pulse waveform is matched with the waveform of the harmonic waveactually outputted. As described, by comparing the waveform of thecurrent pulse with the waveform of the harmonic wave output by theelement temperature fine adjustment section, it can be observed to whichdirection (lower temperature side or higher temperature side), thetemperature of the wavelength conversion element is shifted from thephase matching temperature. Therefore, by carrying out a fine adjustmenton the element temperature by the element temperature fine adjustmentsection so that the current pulse waveform is matched with the outputwaveform of the harmonic wave, it is possible to appropriately set thetemperature of the wavelength conversion element to the phase matchingtemperature.

With the foregoing structure, it is preferable that the harmonic waveresulting from the wavelength conversion by the wavelength conversionelement has a wavelength which is less than 2 times of a wavelength ofan absorption edge of nonlinear optical crystals of the wavelengthconversion element.

The foregoing conditions are appropriate conditions for the wavelengthconversion element made of a material whose light absorption propertiesvary according to the output level of the harmonic wave.

With the foregoing structure, it is preferable that the nonlinearoptical crystals of the wavelength conversion element is LN: lithiumniobate (including those having magnesium oxide added), lithiumtantalate (including those having magnesium oxide added), KTiOPO₄ (KTP:Potassium Titanyl Phosphate), and the wavelength conversion elementemits a green light having a wavelength in a range of from 520 nm to 560nm as a result from the wavelength conversion.

With the foregoing structure, it is preferable that the wavelengthconversion element has a beam diameter variable structure for varying abeam diameter of a harmonic wave outputted from the wavelengthconversion element, the wavelength conversion laser light source furtherincluding a beam shape abnormality detection mechanism for detecting anabnormality in beam shape of the harmonic wave outputted from thewavelength conversion element.

According to the foregoing structure, the wavelength conversion elementhas a beam diameter variable structure for varying a beam diameter of aharmonic wave, it is possible to detect an abnormality in the beam shapeof the harmonic wave by the beam shape abnormality detection mechanism.

With the foregoing structure, it is preferable that the beam shapeabnormality detection mechanism includes a photoreceptor which monitorsan intensity of the harmonic wave outputted from the wavelengthconversion element; and the photoreceptor includes a beam diameterdetection mechanism for detecting an abnormality in beam diameter of theharmonic wave outputted from the wavelength conversion element.

With the foregoing structure, it is preferable that the wavelengthconversion element having the beam diameter variable structure is MgO:LiNbO₃ having a periodical polarization inversion structure; and theharmonic wave has an output power of not higher than 3 W. In the case ofadopting the MgO: LiNbO₃, the beam becomes in a doughnut shape, and theharmonic wave output becomes unstable at a point the output of thesecond harmonic wave exceeding 3 W. However, in the output range of nothigher than 3 W of the harmonic wave, an abnormality in beam shape doesnot occur.

With the foregoing structure, it is preferable that the wavelengthconversion element having the beam diameter variable structure is a MgO:LiTaO₃ crystal element having a periodical polarization inversionstructure; and the harmonic wave has an output power of less than 6.5 W.In the case of adopting the MgO: LiTaO₃, in the output range of notlarger than 6.5 W of the harmonic wave, an abnormality in beam shapedoes not occur.

With the foregoing structure, it is preferable that the beam diameterdetection mechanism includes a photodiode divided into segments.

According to the foregoing structure, by the photodiode divided intosegments, it is possible to detect an abnormality in beam shape(doughnut shape) of the harmonic wave output.

With the foregoing structure, it is preferable that the beam diameterdetection mechanism is divided into two segments A and B; and the beamshape abnormality detection mechanism determines that an abnormalityoccurs in beam diameter of harmonic wave when satisfying the conditionof:

d×LA<LB(0≦d≦0.5),

wherein a segment A is a segment to be irradiated with the harmonic wavein a normal state, a segment B is a segment to be irradiated with theharmonic wave in an abnormal state, LA is an amount of the harmonic waveirradiated in the segment A, and LB is an amount of the harmonic waveirradiated in the segment B.

According to the foregoing structure, in an event that an abnormality inbeam shape (doughnut shape) of the harmonic wave output occurs, theamount of harmonic wave received by the segment B would increase and theabove condition is satisfied. It is therefore possible to accuratelydetect an occurrence of an abnormality in beam shape (beam diameter).

A wavelength conversion laser light source according to another aspectof the present invention includes: a fundamental wave laser lightsource; a wavelength conversion element for converting a fundamentalwave emitted from the fundamental wave laser light source into aharmonic wave, the wavelength conversion element being made of amaterial whose light absorption properties are subjected to changeaccording to an output level of a harmonic wave; an output settingsection for setting a harmonic wave output power level; and an elementtemperature holding section which holds the wavelength conversionelement at an optimal temperature set for a harmonic wave output powerlevel corresponding to 110% to 120% of an upper limit of the outputpower level that can be set in the output setting device.

According to the foregoing structure, in an entire output range for theharmonic wave that can be set, such input/output characteristics thatthe harmonic wave output increases monotonically with an increase in thefundamental wave input can be achieved. Namely, it is possible tocontrol the harmonic wave output with a current value applied to thewavelength conversion laser light source.

With the foregoing structure, it is preferable that the elementtemperature holding section includes a memory for storing thereinelement temperatures to be held by the element temperature holdingsection.

With the foregoing structure, it is preferable that the an elementtemperature learning section be provided for learning a temperature ofthe wavelength conversion element as stored in the memory, to rewritethe currently stored temperature into a temperature as learned; and theelement temperature learning section which searches for an optimalelement temperature at which a harmonic wave of the highest power can beoutputted, with a start temperature for the search, a predeterminedtemperature Δt (° C.) below the temperature of the wavelength conversionelement as stored in the memory, wherein the element temperaturelearning section carries out the search by increasing the temperaturefrom the start temperature, and stores in the memory the optimaltemperature as searched.

According to the foregoing structure, the element temperature learningsection carries out the learning by a peak search for a maximum harmonicwave output value while changing the temperature of the wavelengthconversion element. The element temperature learning section thendetermines a temperature, at which a harmonic wave of the highest powercan be outputted, as an optimal temperature corresponding to theharmonic wave output value. With this structure, even when the optimaltemperature of the wavelength conversion element corresponding to theharmonic wave output value changes from that stored in the memory due tothe deterioration of the wavelength conversion element as time passes,an optimal temperature obtained by carrying out the learning can bestored to be replaced with the currently stored temperature. As aresult, it is possible to perform the frequency conversion at highefficiency over a long period of time.

A two-dimensional image display device in accordance with still anotheraspect of the present invention includes: a wavelength conversion laserlight source of any of the foregoing structure, a two-dimensionalmodulation element that two-dimensionally modulates an output beam fromthe wavelength conversion laser light source; and a projection lens thatprojects the output beam from the two-dimensional modulation element.

According to the foregoing structure, it is possible to realize thetwo-dimensional image display device adopting the wavelength conversionlaser light source which permits a wavelength conversion with highefficiency even when a phase matching temperature of the wavelengthconversion element changes according to the output level of the harmonicwave.

A two-dimensional image display device, in accordance with still anotheraspect of the preset invention includes: the light source unit includingthe wavelength conversion laser light source having any of the foregoingstructures; and a liquid crystal panel which emits the output beam fromthe light source unit.

According to the foregoing structure, it is possible to realize thetwo-dimensional image display device adopting the wavelength conversionlaser light source which permits a wavelength conversion with highefficiency even when a phase matching temperature of the wavelengthconversion element changes according to the output level of the harmonicwave.

A laser light source device, in accordance with still another aspect ofthe preset invention includes: the wavelength conversion laser lightsource having any of the foregoing structure; and a fiber for guidingthe harmonic wave outputted from the wavelength conversion laser lightsource to an irradiation area.

According to the foregoing structure, it is possible to realize thelaser light source device which permits a wavelength conversion withhigh efficiency even when a phase matching temperature of the wavelengthconversion element changes according to the output level of the harmonicwave.

A temperature setting method of a wavelength conversion element inaccordance with still another aspect of the present invention, which ismade of a material whose light absorption properties are subjected tochange according to an output power of a harmonic wave resulting fromconverting the wavelength of a fundamental wave includes the steps of:setting a harmonic wave output level; and switching a temperature of thewavelength conversion element according to an output level of theharmonic wave.

According to the foregoing method, it is possible to realize awavelength conversion with high efficiency even when a phase matchingtemperature of the wavelength conversion element changes according tothe output level of the harmonic wave.

A temperature setting method of a wavelength conversion element inaccordance with still another aspect of the present invention, which ismade of a material whose light absorption properties are subjected tochange according to an output power of a harmonic wave resulting fromconverting the wavelength of a fundamental wave, includes the steps of:setting a harmonic wave output level; and setting an optimal temperatureof the wavelength conversion element to a temperature at which aharmonic wave of an output level of 110% to 120% of an upper limit ofthe output level that can be set in the output setting device.

According to the foregoing method, it is possible to realize awavelength conversion with high efficiency even when a phase matchingtemperature of the wavelength conversion element changes according tothe output level of the harmonic wave.

Although the present invention has been fully described by way ofexample with reference to the accompanying drawings, it is to beunderstood that various changes and modifications will be apparent tothose skilled in the art. Therefore, unless otherwise such changes andmodifications depart from the scope of the present invention hereinafterdefined, they should be construed as being included therein.

1. A wavelength conversion laser light source, comprising: a fundamentalwave laser light source; a wavelength conversion element for convertinga fundamental wave emitted from the fundamental wave laser light sourceinto a harmonic wave, the wavelength conversion element being made of amaterial whose light absorption properties are subjected to changeaccording to an output level of a harmonic wave; an output settingsection for setting a harmonic wave output power value; and an elementtemperature switching section that switches a temperature of thewavelength conversion element according to a harmonic wave output levelas set in the output setting section, wherein said element temperatureswitch section includes an element temperature holding section thatholds said wavelength conversion element at the temperature as switchedby said element temperature switching section, said wavelengthconversion element has a beam diameter variable structure for varying abeam diameter of a harmonic wave outputted from said wavelengthconversion element, said wavelength conversion laser light sourcefurther comprising: a beam shape abnormality detection mechanism fordetecting an abnormality in beam shape of the harmonic wave outputtedfrom said wavelength conversion element.
 2. The wavelength conversionlaser light source according to claim 1, wherein: said beam shapeabnormality detection mechanism includes a photoreceptor that monitorsan intensity of the harmonic wave outputted from said wavelengthconversion element; and said photoreceptor includes a beam diameterdetection mechanism for detecting an abnormality in beam diameter of theharmonic wave outputted from said wavelength conversion element.
 3. Thewavelength conversion laser light source according to claim 1, wherein:said wavelength conversion element having said beam diameter variablestructure is a MgO: LiNbO₃ having a periodical polarization inversionstructure; and said harmonic wave has an output power of not higher than3 W.
 4. The wavelength conversion laser light source according to claim1, wherein: said wavelength conversion element having said beam diametervariable structure a MgO: LiTaO₃ crystal element having a periodicalpolarization inversion structure; and said harmonic wave has an outputpower of less than 6.5 W.
 5. The wavelength conversion laser lightsource according to claim 2, wherein: said beam diameter detectionmechanism includes a photodiode divided into segments.
 6. The wavelengthconversion laser light source according to claim 5, wherein: said beamdiameter detection mechanism is divided into two segments A and B; andsaid beam shape abnormality detection mechanism determines that anabnormality occurs in beam diameter of harmonic wave when satisfying thecondition of:d×LA<LB(0≦d≦0.5), wherein the segment A is a segment to be irradiatedwith the harmonic wave in a normal state, the segment B is a segment tobe irradiated with the harmonic wave in an abnormal state, LA is anamount of the harmonic wave irradiated in the segment A, and LB is anamount of the harmonic wave irradiated in the segment B.
 7. Atwo-dimensional image display device, comprising: a wavelengthconversion laser light source of claim 1; a two-dimensional modulationelement that two-dimensionally modulates an output beam from saidwavelength conversion laser light source; and a projection lens thatprojects the output beam from said two-dimensional modulation element.8. A two-dimensional image display device, comprising: a light sourceunit including said wavelength conversion laser light source as setforth in claim 1; and a liquid crystal panel that is illuminated byoutput light emitted from said light source unit.
 9. A laser lightsource device, comprising: said wavelength conversion laser light sourceof claim 1; and a fiber for guiding the harmonic wave outputted fromsaid wavelength conversion laser light source to an irradiation area.